COMETS AND SHOOTING STARS

Star-land: Being Talks With Young People About the Wonders of the Heavens by Robert S. Ball is part of the HackerNoon Books Series. You can jump to any chapter in this book here. COMETS AND SHOOTING STARS

LECTURE V. COMETS AND SHOOTING STARS.

The Movements of a Comet—Encke’s Comet—The Great Comet of Halley—How the Telegraph is used for Comets—The Parabola—The Materials of a Comet—Meteors—What becomes of the Shooting Stars—Grand Meteors—The Great November Showers—Other Great Showers—Meteorites.

THE MOVEMENTS OF A COMET.

The planets are all massive globes, more or less flattened at the Poles; but now we have to talk about a multitude of objects of the most irregular shapes, and of the most flimsy description. We call them comets, and they exist in such numbers that an old astronomer has said “there were more comets in the sky than fishes in the sea,” though I think we cannot quite believe him. There is also another wide difference between planets and comets: planets move round in nearly circular ellipses, and not only do we know where a planet is to-night, but we know where it was a month ago, or a hundred years ago, or where it will be in a hundred years or a thousand years to come. All such movements are conducted with conspicuous regularity and order; but now we are to speak of bodies which generally come in upon us in the most uncertain and irregular fashion. They visit us we hardly know whence, except that it is from outer space, and they are adorned in a glittering raiment, almost spiritual in its texture. They are always changing their appearance in a baffling, but still very fascinating manner. If an artist tries to draw a comet, he will have hardly finished his picture of it in one charming robe before he finds it arrayed in another. The astronomer has also his complaints to make against the comets. I have told you how thoroughly we can rely on the movements of the planets, but comets often play sad pranks with our calculations. They sometimes take the astronomers by surprise, and blaze out with their long tails just when we do not expect them. Then by way of compensation they frequently disappoint us by not appearing when they have been most anxiously looked for.

After a voyage through space the comet at length begins to draw in towards the central parts of our system, and as it approaches the sun, its pace becomes gradually greater and greater; in fact, as the body sweeps round the sun the speed is sometimes 20,000 times faster than that of an express train. It is sometimes more than 1000 times as fast as the swiftest of rifle bullets, occasionally attaining the rate of 200 miles a second. The closer the comet goes to the sun, the faster it moves; and a case has been known in which a comet, after coming in for an incalculable duration of time towards the sun, has acquired a speed so tremendous, that in two hours it has whirled round the sun and has commenced to return to the depths of outer space. This terrific outburst of speed does not last long. A pace which near the sun is 20,000 times that of our express trains diminishes to 10,000 times, to fifty times, to ten times that pace; while in the outermost part of its path the comet seems to creep along so slowly that we might think it had been fatigued by its previous exertions.

Fig. 72.—How the Comet’s Tail is disposed.

We have so often seen a stream of sparks stretching out along the track of a sky-rocket, that we might naturally suppose the tail of a comet streamed out along its path in a somewhat similar manner. This would be quite wrong. You see from Fig. 72 that the tail does not lie along the comet’s path, but is always directed outwards from the sun. If you will draw a line from the sun to the head of the comet and follow the direction of the line, it shows the way in which the tail is arranged. You will also notice how the tail of the comet seems to grow in length as it approaches the sun. When the comet is first seen, the tail is often a very insignificant affair, but it shoots out with enormous rapidity until it becomes many millions of miles258 long by the time the comet is whirling round the sun. Those glories soon begin to wane as the comet flies outward; the tail gradually vanishes, and the wanderer retreats again to the depths of space in the same undecorated condition as that in which it first approached.

When a comet appears, it is always a matter of interest to see whether it is an entirely new object, or whether it may not be only another return of a comet which has paid us one or more previous visits. The question then arises as to how they are to be identified. Here we see a wide contrast between unsubstantial bodies like comets and the weighty and stately planets. Sketches of the various planets or of the face of the sun, though they might show slight differences from time to time, are still always sufficiently characteristic, just as a photographic portrait will identify the individual, even though the lapse of years will bring some changes in his appearance. But the drawing of a comet is almost useless for identification. You might as well try to identify a cloud or a puff of smoke by making a picture of it. Make a drawing of a comet at one appearance, and sketch particularly the ample tail with which it is provided. The next time the comet comes round it may very possibly have two tails, or possibly no tail at all. We are therefore unable to place any reliance on the comet’s personal appearance in our efforts to identify it. The highway which it follows through the sky affords the only means of recognition; for the comet, if undisturbed by other objects, will never change its actual orbit. But even this method of identification often fails, for it not unfrequently happens that during its259 erratic movements the comet gets into fearful trouble with other heavenly bodies. In such cases the poor comet is sometimes driven so completely out of its road that it has to make for itself an entirely new path, and our efforts to identify it are plunged in confusion. It has happened that a second comet or even a third will be found in nearly the same track, but whether these are wholly different, or whether they are merely parts of the same original object, it is often impossible to determine.

The great majority of comets are only to be seen with a telescope, and hardly a year passes without the detection of at least a few of these faint objects. The number of really brilliant comets that can be seen in a lifetime could, however, be counted on the fingers.

ENCKE’S COMET.

We have already alluded to a little body called Encke’s comet, which was discovered by an astronomer at Marseilles. It was in the year 1818 that he was scanning the heavens with a small telescope, when an object attracted his attention. It was not one of those grand long-tailed comets which every one notices; this body was so faint that it merely appeared as a very small cloud of light, and was recognized as a comet by the fact that it was moving. It happens that there are other bodies in the sky very like comets; we call them nebulæ, and we shall have something to say about them afterwards. But it is remarkable that just as a planet is liable to be mistaken for a star, so a comet is liable260 to be mistaken for a nebula. However, in each case the fact of its movement is the test by which the planet or the comet is at once detected. A nebula stays always in the same spot, like a star, while a comet is incessantly moving. In fact, with a telescope you can actually watch a comet stealing past the stars that lie near it. You know that an object a very long way off may appear to move slowly, though in reality it is moving very rapidly. Look at a steamer near the horizon at sea. In the course of a minute or two it will not appear to have shifted its position to any appreciable extent, but that is because it is far off. If you were near the ship, you would see that it was dashing along at the rate of perhaps fifteen or twenty miles an hour. In a similar manner the comet seems to move slowly, because it is at such a great distance. As a matter of fact it is moving faster at the time we see it than any steamer, faster than any express train, faster than any cannon-ball. There were special reasons why the movements of Encke’s comet should be watched with peculiar care, and the track which it pursued be ascertained. If you can observe a comet three times and measure its position in the sky, the movement of that comet is completely determined. Perhaps I should say would be determined if the comet were let alone, which, unfortunately, is not often the case. Indeed, you may remember how I told you some of the misadventures of this very comet when we were speaking about the planet Mercury. Encke’s comet comes round in a period of a little more than three years, and it gives us some curious information that has been ascertained during261 its journeys. One of the facts we have thus learned is so important that we cannot omit to notice it (Fig. 73).

Fig. 73.—The Orbit of Encke’s Comet.

At increasing heights above the earth’s surface there is gradually less and less air; until at last, at about 200 or 300 miles above the surface on which we dwell, there would be none. You might as well try to quench your thirst by drinking out of an empty cup as attempt262 to breathe in the open space which begins a few hundred miles aloft. In open space motion could take place quite freely. Down here the resistance of the air is a great impediment to movement, especially when very rapid. A heavy cannon-ball is checked and robbed of its pace by having to plough its way through our dense atmosphere. The motion is arrested in the same way, though not of course to the same degree, as if the cannon-ball had been fired into water. Unsubstantial objects are, of course, impeded by the air to a far greater extent than such heavy bodies as cannon-balls. You know that you cannot throw a handful of feathers across the road in the same way that you could throw a handful of gravel. The light feathers cannot force their way through the air so well as the pebbles. A body so flimsy as a comet would never be able to push its way through an atmosphere like ours; but out in empty space the comet meets with no resistance during the greater part of its path. Accordingly, though it has little more substance than a will-o’-the-wisp, the comet pursues its journey with as much resolute dignity as if it were made of cast iron. If in any part of its track the body should have to pierce its way through any material like even the thinnest possible air, then the unsubstantial nature of the cometary materials would be at once shown. The motion would be impeded, and the body’s path would be changed. In this way a comet may be made very instructive, for it will show whether space is really so empty as we sometimes suppose it to be. During the greater part of its course the flimsy little Encke tears along with such ease and speed that263 there seems to be nothing to impede it, and thus we learn that space is generally empty. However, when the comet begins to wheel around the sun, the freedom of its movements seems to receive a check. The unsubstantial object has to force its way with a difficulty that it did not experience so long as it was moving round the greater part of its orbit. We thus learn that there is a thin diffused atmosphere surrounding the sun. We cannot, indeed, say that it is like our air. Its composition is quite different, and almost the only way we know of its existence is by the evidence which this comet affords. In a former lecture I showed how Encke’s comet told us the mass of the planet Mercury. Now we see how the travels of the same body give us information about the sun himself. I ought, however, to add that some more recent observations seem not to have confirmed the belief that there is the resistance of the kind we have just been considering.

THE GREAT COMET OF HALLEY.

I dare say you would think it more interesting to talk about some big and bright comets rather than about objects so faint as that of Encke. It unfortunately happens that most of the fine comets pay our system only a single visit. There is only one of the really splendid objects of this kind that comes back to us with anything like regularity.

It was last seen in the year 1835, and I am glad to tell you that it is coming again; it is expected about the year 1910. You may ask, How can we feel sure264 that such a prediction as I have mentioned will turn out correctly? The fact is that this comet has been watched for a great many centuries. We find ancient records, some of them nearly 2000 years old, of the appearance of grand comets, and several of these are found to fit in with the supposition that there is a body which accomplishes its journey in a period of about seventy-five or seventy-six years. Of course there are thousands of other comets recorded in these old books as well; but what I mean is that among the records many are found which clearly indicate some successive returns of this particular body.

I will explain how the movements of this comet were discovered. There was a great astronomer called Halley, who lived two hundred years ago, and in the year 1682 he, like every one else, was looking with admiration at a splendid comet with a magnificent tail which adorned the sky in that year. At the observatories, of course, they diligently set down the positions of the comet, which they ascertained by carefully measuring it with telescopes. Halley first calculated the highway which this comet followed through the heavens, and then he looked at the list of old comets that had been seen before. He thus found that in 1607—that was, seventy-five years earlier—a great comet had also appeared, the path of which seemed much the same as that which he found for the body that he had himself observed. This was a remarkable fact, and it became still more significant when he discovered that seventy-six years earlier—namely, in 1531—another great comet had been recorded, which265 moved in a path also agreeing with those of 1607 and 1682. It then occurred to Halley that possibly these were not three different objects, but only different exhibitions of one and the same, which moved round in the period of seventy-five or seventy-six years.

There is a test which an astronomer can often apply in the proof of his theory, and it is a very severe test. He will not only show himself to be wrong if it fails, but he will also make himself somewhat ridiculous. Halley ventured to submit his reputation to this ordeal. He prophesied that the comet would appear again in another seventy-five or seventy-six years. He knew that he would, of course, be dead long before 1758 should arrive; but when he ventured to make the prediction, he said that he hoped posterity would not refuse to admit that this discovery had been made by an Englishman.

You can easily imagine that as 1758 drew near, great interest was excited among astronomers to see if the prediction of Halley would be fulfilled. We are accustomed in these days to find many astronomical events foretold with the same sort of punctuality as we expect in railway time-tables. The Nautical Almanac is full of such prophecies, and we find them universally fulfilled. Even now, however, we are not able to set forth our time-tables for comets with the same confidence that we show when issuing them for the sun, the moon, or the stars. How astonishing, then, must Halley’s prediction have seemed! Here was a vast comet which had to make a voyage through space to the extent of many hundreds of millions of miles. For three-quarters266 of a century it would be utterly invisible in the greatest telescopes, and the only way in which it could be perceived was by figures and calculations which enabled the mind’s eye to follow the hidden body all around its mysterious track. For fifty, or sixty, or seventy years nothing had been seen of the comet, nor, indeed, was anything expected to be seen of it; but as seventy-one, and seventy-two, and seventy-three years had passed, it was felt that the wanderer, though still unseen, must be rapidly drawing near. The problem was made more difficult for those skilful mathematicians who essayed to calculate it by the fact that the comet approached the thoroughfares where the planets circulate; and, of course, the flimsy object would be pulled hither and thither out of its path by the attractions of the weighty bodies. It was computed that the influence of Saturn alone was sufficient to delay the comet for more than three months, while it appeared that the attraction of Jupiter was potent enough to retard the expected event for a year and a half more. Was it not wonderful that mathematicians should be able to find out all these facts from merely knowing the track which the comet was expected to follow? Clairaut, who devoted himself to this problem, suggested that there might also be some disturbances from other causes of which he did not know, and that consequently the expected return of the comet might be a month wrong either way. Great indeed was the admiration in astronomical circles when, true to prediction, the comet blazed upon the world within the limits of time Clairaut had specified.

267The remarkable fulfilment of this prophecy entitles us to speak with confidence about the past performances of this comet. Among all the apparitions of Halley’s comet for the last two thousand years, perhaps the most remarkable is that which took place in the year 1066. I am sure you will all remember this date in your English history; it was the year of the Conquest. In those days they did not understand astronomy as we understand it now; they used to think of a comet as a fearful portent of evil, sent to threaten some frightful calamity; such as a pestilence, a war, a famine, or something else equally disagreeable. Hence in the year of the Conquest the appearance of so terrific an object in the sky was a very significant omen. Attention was concentrated upon the spectacle, and a picture of Halley’s comet as it appeared to the somewhat terrified imaginations of the people of those days has been preserved. There is a celebrated tapestry at Bayeux on which historical incidents are represented by beautifully worked pictures. On this fabric we have a view of Halley’s comet in a quaint and rather ludicrous aspect. You will read of this comet also in the early pages of Tennyson’s “Harold.”

HOW THE TELEGRAPH IS USED FOR COMETS.

In these days the study of comets is prosecuted with energy. Over the world observatories are situated, and whenever a comet is discovered, tidings of the event are diffused among those likely to be interested. Suppose that one is discovered in the southern hemisphere,268 the astronomers then write to warn the northern observatories of the event. But comets often move faster than her Majesty’s mails, so that the telegraph has to be put into requisition. The kind of message is one which shall show the position and the movements of the body. It necessarily involves a good many figures and words, and consequently it is desirable to abbreviate as much as possible for the sake of economy. There is a further difficulty in using the telegraph, because the messages are not of an intelligible description to those not specially versed in astronomy. Skilful as the telegraph clerks are, they can hardly be expected to be familiar with the technicalities of astronomers. The clerk at the receiving end is handed a message which he does not understand very clearly. The clerk at the other end does not understand the message which is delivered to him, and between them it has happened that they have transformed the message into something which not only they do not understand, but which, unfortunately, nobody else can understand either. These difficulties have been surmounted by an agreement between astronomers, which is so simple and interesting that I must mention it.

The kind of message that expresses the place of a comet will consist of sentences something of this kind: “One hundred and twenty-three degrees and forty-five minutes.” Surely it would be an advantage to be able to replace all these words by a single word, particularly if by doing so the risk of error would be diminished. This is what the astronomers’ telegraphic arrangement enables them to accomplish. There is a269 certain excellent Dictionary known as Worcester’s. I am sure when Mr. Worcester arranged this work, he had not the slightest anticipation of an odd use to which it would occasionally be put. Every astronomer who is co-operating in the comet scheme must have a copy of the book. To send the message I have just referred to, he has to take up his Dictionary and look out page 123. Then he will count down the column until he comes to the forty-fifth word on that page, which he finds to be “constituent,” and according to this plan the message, or at least this part of it, is merely that one word, “constituent.” The astronomer who receives this message and wishes to interpret it takes up his copy of Worcester’s Dictionary and looks out for “constituent.” He sees that it is on page 123, and that it is the forty-fifth word down on that page; and therefore he knows that the interpretation of the message is to be one hundred and twenty-three degrees and forty-five minutes.

THE PARABOLA.

Generally speaking, great comets come to us once and are then never seen again. Such bodies do not move in closed ovals or ellipses, they follow another kind of curve, like that represented in Fig. 74. It is one that every boy ought to know. In fact, in one of his earliest accomplishments he learned how to make a parabola. It is true he did not call it by any name so fine as this, but every time a ball is thrown into the air it describes a part of the beautiful curve which geometers know by this word (Fig. 74). In fact, you could270 not throw a ball so that it should describe any other curve except a parabola. No boy could throw a stone in a truly horizontal line. It will always curve down a little, will always, in fact, be a portion of a parabola.

Fig. 74.—The Path of a Projectile is a Parabola.

There are big parabolas and there are small ones. One of the shells which are thrown into a town when bombarded from a distance describes, as it rises and then slopes down again, part of a mighty parabola. So does a tennis ball thrown by the hand or struck by the racket; though here, indeed, I admit that a spin may be given to the ball which will somewhat detract from the simplicity of its movement. In playing baseball, a large part of the skill of the pitcher consists in throwing the ball in such a way that it shall not move in a parabola, but in some twisting curve by which he hopes to baffle his adversary. Setting aside these exceptions, and such another as the case of a body tossed straight up or271 dropped straight down, we may assert that the path of a projectile is a parabola.

Fig. 75.—The Lighthouse Reflector.

There are some remarkable applications of the same curve for practical purposes. From our lighthouses we want to send beams off to sea, so as to guide ships into port. If we merely employed a lamp without concentrating its rays, we should have a very imperfect lighthouse, for the lamp scatters light about in all directions. Much of it goes straight up into the air, much of it would be directed inland; in fact, there is only an extremely small part of the entire number of rays that will naturally take the useful direction. We therefore require something round the lamp which shall catch the truant rays that are running away to idleness and loss, and shall concentrate them into the direction in272 which they will be useful to the mariner. An effective way of doing this is to furnish the lamp with a reflector. On its bright surface (Fig. 75) all the rays fall which would otherwise have gone astray, and each of them is properly redirected, where the sailors can see it. It is essential that the mirror shall do this work accurately, and this it will only do when it has been truly shaped so as to be a parabola.

You will remember, also, how I described to you the reflector which Herschel made for his great telescope. The shape of the mirror must be most accurately worked, and it, too, must have a parabola for its section; so that you see this curve is one of importance in a variety of ways.

But the grandest of all parabolas are those which the comets pursue. Unlike the ellipse, the parabola is an open curve; it has two branches stretching away and away forever, and always getting further apart. Of course, in the examples of this curve that I have given it is only a small part of the figure that is concerned. When you throw a stone it only describes that part of the parabola that lies between your hand and the spot where the stone hits the ground. It is just a part of the curve in the same way that a crescent may be a bit of a circle. It is to comets that we must look for the most complete illustration of the ample extent of a parabola.

The shape of this grand curve will explain why so many comets only appear to us once. It is quite clear that if you begin to run round a closed racecourse, you may, if you continue your career long enough, pass273 and repass the starting-post thousands of times. Thus, comets which move in ellipses, and are consequently tracing closed curves, will pass the earth times without number. For this reason we may see them over and over again, as we do Encke’s comet or Halley’s comet. But suppose you were travelling along a road which, no matter how it may turn, never leads again into itself, then it is quite plain that, even if you were to continue your journey forever, you can never twice pass the same house on the roadside. That is exactly the condition in which most of the comets are moving. Their orbits are parabolas which bend round the sun; and, generally speaking, the sun is very close to the turning-point. The earth is also, comparatively speaking, close to the sun; so that while the comet is in that neighborhood we can sometimes see it. We do not see the comet for a long time before it approaches the sun, or for a long time after it has passed the sun. All we know, therefore, of its journey is that the two ends of the parabola stretch on and on forever into space. The comet is first perceived coming in along one of these branches to whirl round the sun; and after doing so, it retreats along the other branch, and gradually sinks into the depths of space.

Why one of these mysterious wanderers should approach in such a hurry, and then why it should fly back again, can be partially explained without the aid of mathematics.

Let us suppose that, at a distance of thousands of millions of miles, there floated a mass of flimsy material resembling that from which comets are made. Notwithstanding274 its vast distance from the sun, the attraction of that great body will extend thither. It is true the pull of the sun on the comet will be of the feeblest and slightest description, on account of the enormously great distance. Still, the comet will respond in some degree, and will commence gradually to move in the direction in which the sun invites it. Perhaps centuries, or perhaps thousands, or even tens of thousands, of years will elapse before the object has gained the solar system. By that time its speed will be augmented to such a degree, that after a terrific whirl around the sun, it will at once fly off again along the other branch of the parabola. Perhaps you will wonder why it does not tumble straight into the sun. It would do so, no doubt, if it started at first from a position of rest; generally, however, the comet has a motion to begin with which would not be directed exactly to the luminary. This it is which causes the comet to miss actually hitting the sun.

It may also be difficult to understand why the sun does not keep the comet when at last it has arrived. Why should the wandering body be in such a hurry to recede? Surely it might be expected that the attraction of the sun ought to hold it. If something were to check the pace of the comet in its terrific dash round the sun, then, no doubt, the object would simply tumble down into the sun and be lost. The sun has, however, not time to pull in the comet when it comes up with a speed 20,000 times that of an express train. But the sun does succeed in altering the direction of the motion of the comet, and the attraction has shown itself in that way.

275I can illustrate what happens in this manner. Here is a heavy weight suspended from the ceiling by a wire; it hangs straight down, of course, and there it is kept by the pull of the earth. Supposing I draw the weight aside and allow it to swing to and fro, then the motion continues like the beat of a pendulum. The weight is always pulled down as near to the earth as possible, but when it gets to the lowest point, it does not stay there, it goes through that point, and rises up at the other side. The reason is that the weight has acquired speed by the time it reaches the lowest point; and that, in virtue of its speed, it passes through the position in which it would naturally rest, and actually ascends the other side in opposition to the earth’s pull, which is dragging it back all the time. This will illustrate how the comet can pass by and even recede from the body which is continually attracting it.

Just a few words of caution must be added. Suppose you had an ellipse so long that the comet would take thousands and thousands of years to complete a circuit, then the part of the ellipse in which the comet moves during the time when we can see it is so like a parabola, that we might possibly be mistaken in the matter. In fact, a geometer will tell us that if one end of an ellipse was to go further and further away, the end that stayed with us would gradually become more and more like this curve. Therefore, some of those comets which seem to move in parabolas may really be moving in extremely elongated ellipses, and thus, after excessively long periods of time, may come back to revisit us.

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THE MATERIALS OF A COMET.

A comet is made of very unsubstantial material. This we can show in a very interesting manner, when we see it moving over the sky between the earth and the stars. Sometimes a comet will pass over a cluster of very small stars, so faint that the very lightest cloud that is ever in the sky would be quite sufficient to hide them. Yet the stars are distinctly visible right through the comet, notwithstanding that it may be hundreds of thousands of miles thick. This shows us how excessively flimsy is the substance of a comet, for while a few feet of haze or mist suffice to extinguish the brightest of stars, this immense curtain of comet stuff, whatever it may be made of, is practically transparent.

I have often told you that we are able to weigh the heavenly bodies, but a comet gives us a great deal of trouble. You see that the weighing machine must be of a very delicate kind if you are going to weigh a very light object. Take, for example, a little lock of golden hair, which no doubt has generally a value quite independent of the number of grains that it contains. Suppose, however, that we are so curious as to desire to know its weight, then one of those beautiful balances in our laboratories will tell us. In fact, if you snipped a little fragment from a single hair, the balance would be sensitive enough to weigh it. If, however, you were only provided with a common pair of scales like those which are suited for the parcel post, then you could never weigh anything so light as a lock of hair. You have not small enough weights to begin with, and even277 if you had they would be of no use, for the scale is too coarse to estimate such a trifle. This is precisely the sort of difficulty we experience when we try to weigh a comet. The body, though so big, is very light, and our scales are so cumbersome that we are in a position of one who would try to weigh a lock of hair with a parcel-post balance. We cannot always find suitable scales for weighing celestial bodies. We have to use for the purpose whatever methods of discovering the weights happen to be available. So far, the methods I have mentioned are of the rudest description; they serve well enough for weighing heavy masses like planets, but they will not do for such unsubstantial bodies as comets.

But, though we fail in this endeavor, i.e. to weigh comets, yet skilful astronomers have succeeded in something which at first you might think to be almost impossible. They have actually been able to discover some of the ingredients of which a comet is made. This is so important a subject that I must explain it fully.

The most instructive comet which we have seen in modern days is that which appeared in the year 1882. It was an object so great that its tail alone was double as long as from the earth to the sun. It was discovered at the observatories in the southern hemisphere early in September of that year. A little later it was observed in the northern hemisphere in extraordinary circumstances. It must be remembered that a comet is generally a faint object, and that even those comets which are large enough and bright enough to form glorious278 spectacles in the sky at night are usually invisible during the brightness of day. For a comet to be seen in daylight was indeed an unusual occurrence; but on the forenoon of Sunday, September 17, Mr. Common at Ealing saw a great comet close to the sun. Unfortunately clouds intervened, and he was prevented from observing the critical occurrence just approaching. An astronomer at the Cape of Good Hope—Mr. Finlay—who had also been one of the earliest discoverers of the comet, was watching the body on the same day. He followed it as it advanced close up to the sun; bright indeed must that comet have been which permitted such a wonderful observation. At the sun’s edge the comet disappeared instantly; in fact, the observers thought that it must have gone behind the sun. They could not otherwise account for the suddenness with which it vanished. This was not what really happened. It was afterwards ascertained that the comet had not passed behind the sun; it had, indeed, come between us and our luminary. In its further progress this body showed in a striking degree the incoherent nature of the materials of which a comet is composed. It seemed to throw off portions of its mass along its track, each of which continued an independent journey. Even the central part in the head of the comet—the nucleus, as it is called—showed itself to be of a widely different nature from a substantial planetary body. The nucleus divided into two, three, four, or even five distinct parts, which seemed, in the words of one observer, to be connected together like pearls on a string.

The comet of 1882 was also very instructive with279 regard to the actual materials from which such bodies are made. Astronomers have a beautiful method by which they find out the substances present in a heavenly body, even though they never can get a specimen of the body into their hands. We know at least three materials which were present in this comet. The first of them is an ingredient which is very commonly found in comets—a chemist calls it carbon. It is an extremely familiar material on the earth; for instance, coal is chiefly composed of carbon. Charcoal when burned leaves only a few ashes. All the substance that has vanished during combustion is carbon; in fact, it is not too much to say that carbon is found abundantly not only in wood, but in almost every form of vegetable matter. The food we eat contains abundant carbon, and it is an important constituent in the building up of our own bodies. Generally speaking, carbon is not found in a pure state—it is associated with other substances. Soot and lampblack are largely composed of it; but the purest form of this element carbon that we know is the diamond.

It is interesting to note that carbon is certainly found as a frequent constituent of comets. The great comet of 1882 undoubtedly contained it, as well as certain other substances. Of these we know two: the first is the element sodium, an extremely abundant material on earth, inasmuch as the salt in the sea is mainly composed of it. It was also discovered that the same great comet contained another substance very common here and extremely useful to mankind. Dr. Copeland and Dr. Lohse at Dunecht showed that iron was present in280 this body which had come in to visit us from the depths of space.

These discoveries are especially interesting because they seem to show the uniformity of material composing our system. We already knew that sodium and iron abounded in the sun, and now we have learned that these bodies and carbon as well are present in the comets. In the next chapter we shall learn that the very same materials—sodium and iron—are met with in bodies far more remote from us than any bodies of our own system.

Comets have such a capricious habit of dashing into the solar system at any time and from any direction, that it is worth while asking whether a comet might not sometimes happen to come into collision with the earth. There is nothing impossible in such an occurrence. There is, however, no reason to apprehend that any disastrous consequences would ensue to the earth. Man has lived on this globe for many, many thousands of years, and the rocks are full of the remains of fossil animals which have flourished during past ages; indeed, we cannot possibly estimate the number of millions of years that have elapsed since living things first crawled about this globe. There has never been any complete break in the succession of life, consequently during all those millions of years we are certain that no such dire calamity has happened to the earth as a frightful collision would have produced, and we need not apprehend any such catastrophe in the future.

I do not mean, however, that harmless collisions with comets may not have occasionally happened; in fact,281 there is good reason for knowing that they have actually taken place. In the year 1861 a fine comet appeared; and it is not so well remembered as its merits deserve, because it happened, unfortunately for its own renown, to appear just three years after the comet of 1858, which was one of the most gorgeous objects of this kind in modern times. But in 1861 we had a novel experience. On a Sunday evening in midsummer of that year, we dashed into the comet, or it dashed into us. We were not, it is true, in collision with its densest part; it was rather the end of the tail which we encountered. There were, fortunately, no very serious results. Indeed, most of us never knew that anything had happened at all, and the rest only learned of the accident long after it was all over. For a couple of hours that night it would seem that we were actually in the tail of the comet, but so far as I know no one was injured or experienced any alarming inconvenience. Indeed, I have only heard of one calamity arising from the collision. A clergyman tells us that at midsummer he was always able in ordinary years to read his sermon at evening service without artificial light. On this particular occasion, however, the sky was overcast with a peculiar glow, while the ordinary light was so much interfered with that the sexton had to provide a pair of candles to enable him to get through the sermon. The expense of those candles was, I believe, the only loss to the earth in consequence of its collision with the comet of 1861.

Fig. 76.—How the Tail of a Comet arises.

The tail of a comet appears to develop under the influence of the sun. As the wandering body approaches282 the source of central heat it grows warm, and as it gets closer and closer to the sun, the fervor becomes greater and greater, until sometimes the comet experiences a heat more violent than any we could produce in our furnaces. The most infusible substances, such as stones or earth, would be heated white-hot and melted, and even driven off into vapor, under the intense heat to which a comet is sometimes exposed. Comets, indeed, have been known to sweep round the sun so closely as to pass within a seventh part of its radius from the surface. It seems that certain materials present in the comet, when heated to this extraordinary temperature, are driven away from the head, and thus form the tail (Fig. 76). Hence we see that the tail consists of a283 stream of vaporous particles, dashing away from the sun as if the heat which had called them into being was a torment from which they were endeavoring to escape.

The tail of a comet is, therefore, not a permanent part of the body. It is more like the smoke from a great chimney. The smoke is being incessantly renewed at one end as the column gets dispersed into the air at the other. As the comet retreats, the sun’s heat loses its power. In the chills of space there is, therefore, no tail-making in progress, while the small mass of the comet renders it unable to gather back again by its attraction the materials which have been expelled. Should it happen that the comet moves in an elliptic orbit, and thus comes back time after time to be invigorated by a good roasting from the sun, it will, of course, endeavor to manufacture a tail each time that it approaches the source of heat. The quantity of material available for the formation of tails is limited to the amount with which the comet originally started; no fresh supply can be added. If, therefore, the comet expends a portion of this every time it comes round, an inevitable consequence seems to follow. Suppose a boy receives a sovereign when he goes back to school, and that every time he passes the pastry-cook’s shop some of his money disappears in a manner that I dare say you can conjecture, I need not tell you that before long the sovereign will have totally vanished. In a similar way comets cannot escape the natural consequences of their extravagance; their store of tail-making substance must, therefore, gradually diminish. At each successive return the tails produced must generally decline in size and284 magnificence, until at last the necessary materials have been all squandered, and we have the pitiful spectacle of a comet without any tail at all.

The gigantic size of comets must excite our astonishment. A pebble tossed into a river would not be more completely engulfed than is our whole earth when it enters the tail of one of these bodies. But we now pass by a sudden transition to speak of the very smallest bodies, of little objects so minute that you could carry them in your waistcoat pocket. You will perhaps be surprised that such things can play an important part in our system and have a momentous connection with mighty comets.

METEORS.

If you look out from your window at the midnight sky, or take a walk on a fine clear night, you will occasionally see a streak of light dash over the heavens, thus forming what is called a falling, or a shooting, star (Fig. 77). It is not really one of the regular stars that has darted from its place. The objects we are now talking of are quite different from stars proper. To begin with, the shooting stars are comparatively close to us when we see them, and they are very small, whereas the stars themselves are enormous globes, far bigger than our earth, or often even bigger than the sun. Sometimes a great shooting star is seen which makes a tremendous blaze of light as bright as the moon, or even brighter still. These objects we call meteors, and you will be very fortunate if you can ever see a really285 fine one. Astronomers cannot predict these things as they predict the appearance of the planets. Bright meteors consequently appear quite unexpectedly, and it is a matter of chance as to who shall enjoy the privilege of beholding them. But it is not about the great meteors that we are now going to speak particularly; they are often not so interesting as the small ones.

Fig. 77.—A Brilliant Meteor.

These little meteoroids, as we shall call them, have a curious history. They become visible to us only at the very last moment of their existence—in fact, the streak of light which forms a shooting star is merely the destruction of a meteoroid. You must always remember that we are here living at the bottom of a great ocean of air, and above the air extends the empty space. Air is a great impediment to motion; a large part of the power of a locomotive engine has to be expended solely in pushing the air out of the way so as to allow286 the train to get through. The faster the speed, the greater is the tax which the air imposes on the moving body. A cannon-ball, for instance, loses an immensity of its speed, and consequently of its power, by having to bore its way through the air. In outer space beyond the limits of this atmosphere, a freedom of movement can be enjoyed of which we know nothing down here. I spoke of this when discussing the movements of Encke’s comet. Even this very unsubstantial body could dash along without appreciable resistance until it traversed the atmosphere surrounding the sun. But now we have to speak of the motion of a little object both small and dense, resembling perhaps a pebble or a fragment of iron, or some substance of that description. It is a little body such as this which produces a shooting star.

For ages and ages the meteoroid has been moving freely through space. The speed with which it dashes along greatly exceeds that of any of the motions with which we are familiar. It is about 100 times as swift as the pace of a rifle-bullet. About twenty miles would be covered in a second. You can hardly imagine what that speed is capable of. Suppose that you put one of these flying meteoroids beside an express train to race from London to Edinburgh, the meteoroid would have won the race before the train had got out of the station. Or suppose that a shooting star determined to make the circuit of the earth, it might, so far as pace is concerned, go entirely around the globe and back to the point from which it started in a little more than twenty minutes. But the fact is, you could not make any object down287 here move as fast as a shooting star. No gunpowder that could be made would be strong enough, in the first place, and even if the body could once receive the speed, it would never be able to force its way through the air uninjured.

So long as a little shooting star is tearing away through open space we are not able to see it. The largest telescope in the world would not reveal a glimpse of anything so small. The meteoroid has no light of its own, and it is not big enough to exhibit the light reflected from the sun in the same manner as a little planet would do. It is only at the moment when it begins to be destroyed that its visibility commences. If the little object can succeed in dashing past our earth without becoming entangled in the atmosphere, then it will pursue its track with perhaps only a slight alteration in its path, due to the pull exercised by the earth. The air which surrounds our globe may be likened to a vast net, in which if any little meteor becomes caught its career is over. For when the little body, after rejoicing in the freedom of open space, dashes into air, immediately it experiences a terrific resistance; it has to force the particles of air out of the way so as to make room for itself, and in doing so it rubs against them with such vehemence that heat is produced.

I am sure every boy knows that if he rubs a button upon a board it becomes very hot, in consequence of the friction. There are many other ways in which we can illustrate the production of heat in the same manner. One is a contrivance by which we spin round rapidly a piece of stick pressed against a board. Quantities288 of heat are thus produced by the friction, and volumes of smoke rise up. We have read how some savages are able to produce fire by means of friction in a somewhat similar manner, but to do so requires a rare amount of skill and patience. There is another illustration by which to show how heat can be produced by friction. A brass tube full of water is so arranged that it can be turned around very rapidly by the whirling table. We apply pressure to the tube, and after a minute or two the water begins to get hot, and then at last to boil, until ultimately the cork is driven out and a diminutive and, fortunately, harmless explosion of the friction boiler takes place. Engineers are aware how frequently heat is produced by friction, when it is very inconvenient or dangerous. Indeed, unless the wheels of railway carriages are kept well greased, the rubbing of the axle may generate so much heat that conflagrations in the carriage will ensue. Nature, in the little shooting star, gives us a striking illustration of the same fact. Perhaps you may be surprised to hear that the whole brilliancy of the shooting star is simply due to friction. As the little body dashes through the air it becomes first red-hot, then white-hot, until at last it is melted and turned into vapor. Thus is formed that glowing streak which we, standing very many miles below, see as a shooting star.

A bullet when fired from a rifle will fly into pieces after it has struck against the target, and if you quickly pick up one of these pieces you will generally find it quite hot. Whence comes this heat? The bullet, of course, was cold before the rifleman pulled the trigger.289 No doubt there was a considerable amount of heat developed by the burning of the gunpowder, but the bullet was so short a time in contact with the wad, through which so little heat would pass, that we must look to some other source for the warmth that has been acquired. Friction against the barrel as the bullet passed to the mouth must have warmed the missile a good deal, and when rubbing against the air the same influence must have added still further to its temperature, while the blow against the target would finally warm it yet more.

In comparing the shooting star with the rifle-bullet we must remember that the celestial object is travelling with a pace 100 times as swift as the utmost velocity that the rifle can produce, and the heat which is generated by friction is increased in still greater proportion. If we double the speed, we shall increase the quantity of heat by friction fourfold; if we increase the speed three times, then friction will be capable of producing nine times as much heat. In fact, we must multiply the number expressing the relative speed by itself—that is, we must form its square—if we want to find an accurate measure for the quantity of heat which friction is able to produce when a rapidly moving body is being stopped by its aid. The shooting star may have a pace 100 times that of the rifle-bullet, and if we multiply 100 by 100 we get 10,000; consequently we see that the heat produced by the shooting star before its motion was arrested in dashing through the air would be 10,000 times that gained by the rifle-bullet in its flight. If the temperature of the rifle-bullet only rose a single290 degree by friction, it would thus be possible for the shooting star to gain 10,000 degrees, and this would be enough to melt and boil away any object which ever existed. Thus we need not be surprised that friction through the air, and friction alone, has proved an adequate cause for the production of all the heat necessary to account for the most brilliant of meteors.

It is rather fortunate for us that the meteors do dash in with this frightful speed; had the little bodies only moved as quickly as a rifle-bullet, or even only four or five times as fast, they would have pelted down on the earth in solid form. Indeed, on rare occasions it does happen that bodies from the heavens strike down on the ground. The great majority of those that fall on the ground, however, become entirely transformed into harmless vapor. The earth would, indeed, be almost uninhabitable from this cause alone were it not for the protection that the air affords us. All day and all night innumerable missiles would be whizzing about us, and though many of them are undoubtedly very small, yet as their speed is 100 times that of a rifle-bullet, the fusillade would be very unpleasant. It is, indeed, the intense hurry of these celestial bullets to get at us which is the very source of our safety. It dissipates the meteors into streaks of harmless vapor.

WHAT BECOMES OF THE SHOOTING STARS.

When we throw a lump of coal on the fire, all that is soon left is a little pinch of ashes, and the rest of the coal has vanished. You might think it had been291 altogether annihilated, but that is not nature’s way. Nothing is ever completely destroyed; it is merely transformed or changed into something else. The greater part of the coal has united with the oxygen which it has obtained from the air, and has formed a new gas, which has ascended the chimney. Every particle that was in the coal can be thus accounted for, and in the act of transformation heat is given out.

A meteor also becomes transformed, but the substance it contains is not lost, though it is changed into glowing vapors. It is known that with heat enough any substance can be turned into vapor, just as water can be boiled into steam. Look at an electric light flashing between two pieces of carbon. Though carbon is one of the most difficult substances to melt, yet such is the intense heat generated by the electric current that the carbon is not only melted, but is actually turned into a vapor, and it is this vapor glowing with heat that gives us the brilliant light. In a similar manner iron can be rendered red-hot, white-hot, and then boiled and transformed into an iron vapor, if we may so call it. There is, indeed, plenty of such iron vapor in the universe. Quantities of it surround the sun and some of the stars.

When ordinary steam is chilled it condenses into little drops of water. So, too, if iron be heated until it is transformed into vapor, and if that vapor be allowed to condense, it will ultimately form a dust, consisting of bits of iron so small that you would require a microscope to examine them. There is iron present in the small shooting stars. Other substances292 are also contained therein, and all these materials, after being boiled by the intense heat, are transformed into vapor. When the heat subsides, the vapor condenses again into fine dust, so that the ultimate effect of the atmosphere on a shooting star is to grind the little object into excessively fine powder, which is scattered along the track which the object has pursued. Sometimes this powder will continue to glow for minutes after the meteor has vanished, and in the case of some great meteors this stream of luminous dust in the air forms a very striking spectacle. A great meteor, or fire-ball as it is often called, appeared on the 6th of November, 1869. It flew over Devonshire and Cornwall, and left a track fifty miles long and four miles wide. The dust remained visible all along the great highway for nearly an hour; it formed a glowing cloud hanging in the sky, and though originally nearly straight, it became bent and twisted by the winds before it finally disappeared from view.

We have now to see what becomes of this meteoric dust which is being incessantly poured into the air from external space. None of it ever gets away again; for whenever an unfortunate meteor just touches the air it is inevitably captured and pulverized. That dust subsides slowly, but we do not find it easy to distinguish the particles which have come from the shooting stars, because there is so much floating dust which has come from other sources.

A sunbeam is the prettiest way of revealing the existence of the motes with which the air is charged. The sunbeam renders these motes visible exactly in the293 same way as planets become visible when sunbeams fall on them in far-distant space. But if we have not the sunbeams here, we can throw across the room a beam of electric light, and it is seen glowing all along its track, simply because the air of the room, like air everywhere, is charged with myriads of small floating particles. If you hold the flame of a spirit-lamp beneath this beam, you will see what seems like columns of black smoke ascending through it. But these columns are not smoke, they are pure air, or rather air in which the solid particles have been transformed into vapor by the heat from the spirit-flame.

The motes abound everywhere in the air. We take thousands of them into our lungs every time we breathe. They are on the whole gradually sinking and subsiding downwards, but they yield to every slightest current, so that when looking at a sunbeam you will find them moving in all directions. It is sometimes hard to believe that the little objects are tending downwards, but if you look on the top of a book that has lain for a time on a book-shelf, you find there a quantity of dust, produced by the motes which have gradually subsided where they found a quiet spot and were allowed sufficient time to do so.

The great majority of these particles consist, no doubt, of fragments of terrestrial objects. The dust from the roads, the smoke from the factories, and numerous other sources, are incessantly adding their objectionable particles to the air. There can be no doubt that the shooting stars also contribute their mites to the dust with which the atmosphere is ever charged. The294 motes in the murky air of our towns have no doubt chiefly originated from sources on this earth. Many of these sources it would be impossible to regard as of a romantic description. We may, however, feel confident that among those teeming myriads of small floating objects are many little particles which, having had their origin from shooting stars, are now gradually sinking to the earth.

This is not a mere surmise, for dust has been collected from lofty Alpine snows, from the depths of the sea, and from other localities far removed from the haunts of men. From such collections, tiny particles of iron have been obtained, which have evidently been once in a molten condition. There is no conceivable explanation for the origin of iron fragments in such situations, except that they have been dropped from shooting stars.

I am sure you have often helped in the making of a gigantic snowball. You begin with a small quantity of snow that can be worked with your hands. Then you have rolled it along the ground until it has become so big and so heavy that you must get a few playmates to help you, until at last it has grown so unwieldy that you can move it no longer, and then you apply your artistic powers to carving out a statue. The snowball has grown by the addition of material to it from without, and as it became heavier and heavier, it lapped up more and more of the snow as it rolled along; so that with each increase of size, its capacity for becoming still larger has also increased. I want to liken our earth to a snowball which goes rolling on through295 space, and every day, every hour, every minute, is gathering up and taking into it the little shooting stars that it meets with on its way. No doubt the annual accumulation is a very small quantity when compared with the whole size of the earth; but the earth is always drawing in, and now, at all events, never giving back again; so that when this process is carried on long enough, astonishing results may be obtained.

You have all heard many maxims on this subject—how every little saving will at length reach a respectable or a gigantic total. Nature abounds with illustrations of the principle. All the water that thunders over Niagara is merely a sufficient number of little drops of rain collected together. Our earth has been gradually hoarding up, during countless ages, all the meteor dust that has rained upon it; and the larger the earth grows, the bigger is the net which it spreads, and the greater is the power it has to capture the wandering bodies. Thus, our earth, ages and ages ago, may have been considerably smaller than it is at present; in fact, a large proportion of this globe on which we dwell may have been derived from the little shooting stars which incessantly rain in upon its surface.

GRAND METEORS.

I dare say that many of those present will, in the course of their lives, have opportunities of seeing some of the great meteors, or fire-balls, which are occasionally displayed. Generally speaking, about one hundred or so of these splendid objects are recorded every year.296 We are never apprised that they are coming; they take us unawares, and therefore we have no opportunity to make proper arrangements for seeing them. There is only the chance that such persons as have been fortunate enough to see them will have noted the circumstances with sufficient accuracy to enable us to make use of their observations.

Fig. 78.—How to find the Height of a Meteor.

The chief point to determine is the height of the meteor above the earth. For this we must have two observations at least, made in places as far asunder as possible. Suppose an observer at London and an observer at York were both witnesses of a splendid meteor; if they find, on subsequent comparison, that their observations were made at the same moment, there is no reasonable doubt that it was the same object they both saw. The observer at York describes the meteor297 as lying to the south, halfway down from the point directly over his head towards the horizon. The London observer speaks of the meteor as being to the north; and to him also it appeared that the object was halfway down towards his horizon from the point directly over his head. If you know a little Euclid, you can easily show from these facts that the height of the meteor must have been half the distance between London and York, that is, 85 miles (Fig. 78).

I do not mean to say that the mode of discovering the meteor’s height will be always quite such a simple process as it has been in the case of the London and York observations. The principle is, however, the same—that whenever from two sufficiently distant positions the direction of the meteor has been observed, its path is known—just as on p. 21 we showed how the height of the suspended ball was obtained from observations at each end of the table. Generally speaking, bright meteors begin at an elevation of between fifty and one hundred miles, and they become extinguished before they are within twenty miles of the ground.

Sometimes a tremendous explosion will take place during the passage of a meteor through the air. There was a celebrated instance in America on the 21st of December, 1876, which will give an idea of one of these objects possessing exceptional magnificence. It began in Kansas about seventy-five miles high, and thence it flew for a thousand miles at a speed of ten or fifteen miles a second, until it disappeared somewhere near Lake Ontario. Over a certain region between Chicago and St.298 Louis, the great ball of fire burst into a number of pieces, and formed a cluster of glowing stars that seemed to chase each other over the sky. This cluster must have been about forty miles long and five miles wide, and when the explosion occurred a most terrific noise was produced, so loud that many thought it was an earthquake. A remarkable circumstance illustrates the tremendous height at which this explosion occurred. The meteor had burst into pieces, the display was all over, and was beginning to be forgotten, and yet nothing had been heard. It was not until a quarter of an hour after the explosion had been seen that a fearful crash was heard at Bloomington. The explosion actually occurred 180 miles from the spot, and as sound takes five seconds to travel a mile, you can easily calculate that the noise required a quarter of an hour for its journey. What a tremendous noise it must have been!

Shooting stars are of every grade of brightness. Beginning with the more gorgeous objects which have been compared with the moon or even with the sun himself, we descend to others as bright as Venus or as Jupiter; others are as bright as stars of various degrees of brilliancy. Fainter shooting stars are much more numerous than the conspicuous ones; in fact, there are multitudes of these objects so extremely feeble that the unaided eye would not show them. They only become perceptible in a telescope. It is not uncommon while watching the heavens at night to notice a faint streak of light dashing across the field of the instrument. This is a shooting star which is invisible except through the telescope.

THE GREAT NOVEMBER SHOWERS.

Fig. 79.—A Great Shower of Shooting Stars.

Occasionally we have the superb spectacle of a shower of shooting stars. None of you, my young friends, can as yet have had the good fortune to witness one of the specially grand displays, but you may live in hope; there are still showers to come. Astronomers have ventured on the prophecy that in or about the year 1899 you will have the opportunity of seeing a magnificent300 exhibition of this kind. There is only one ground for anxiety, and that is as to whether the clouds will keep out of the way for the occasion. I think I cannot explain my subject better than by taking you into my confidence and showing you the reasons on which we base this prediction. The last great shooting-star shower took place in the year 1866, or, perhaps, I should rather say that this was the last display from the same shooting-star system as that about which we are now going to speak. On the night of the 13th of November, 1866, astronomers were everywhere delighted by a superb spectacle. Enjoyment of the wondrous sight was not only for astronomers. Every one who loves to see the great sights of nature will have good reason for remembering that night. I certainly shall never forget it. It was about ten o’clock when a brilliant meteor or two first flashed across the sky, then presently they came in twos and threes, and later on in dozens, in scores, in hundreds. These meteors were brilliant objects, any one of which would have extorted admiration on an ordinary night. What, then, was the splendor of the display when they came on in multitudes? For two or three hours the great shower lasted, and then gradually subsided.

We were not taken unawares on this occasion, for the shower was expected, and had been, in fact, awaited with eager anticipation. It should first be noticed that each year some shooting stars may always be looked for on or about the 13th of November. Every thirty-three years, or thereabouts, the ordinary spectacle breaks out into a magnificent display. It has also been found that301 for nearly 1000 years there have been occasional grand showers of meteors at the time of year mentioned, and all these incidents agree with the supposition that they are merely repetitions of the regular thirty-three-year shower. The first was in the year A.D. 902, which an old chronicle speaks of as the “year of the stars,” from the extraordinary display which then took place. I do not think the good people 1000 years ago fully appreciated the astronomical interest of such spectacles; in fact, they were often frightened out of their wits, and thought the end of the world had come. Doubtless many ancient showers have taken place of which we have no record whatever. In more modern days we have had somewhat fuller information; for example, on the night between the 12th and 13th of November, 1833, a shower was magnificently seen in America. Mr. Kirkwood tells us that a gentleman of South Carolina described the effect on the negroes of his plantation as follows: “I was suddenly awakened by the most distressing cries that ever fell on my ears. Shrieks of horror and cries for mercy I could hear from most of the negroes of the three plantations, amounting in all to about 600 or 800. While earnestly listening for the cause, I heard a faint voice near the door calling my name. I arose, and taking my sword, stood at the door. At this moment I heard the same voice still beseeching me to arise, and crying out that the world was on fire. I then opened the door, and it is difficult to say which excited me the most—the awfulness of the scene or the distressed cries of the negroes. Upwards of a hundred lay prostrate on the ground, some speechless,302 and some with the bitterest cries, but with their hands raised praying for mercy. The scene was truly awful, for never did rain fall much thicker than the meteors fell towards the earth.”

By the study of many records of great showers it was learned that the interval at which these grand displays succeeded one another was about thirty-three years; and when it was remembered that the last great shower was in 1833 it was confidently expected that another similar display would take place in 1866. This was fully confirmed. Yet another thirty-three years brings us to 1899, when we have good reason for looking forward to a grand shower of these bodies. It may be expected to occur on November 14 or November 15. It may, however, possibly be that a shower will occur on the same days of the succeeding year.

Fig. 80.—The Earth crossing the Track of Meteors.

We know a good deal now as regards the movements of these little objects. I want you to think of a vast swarm, something like a flock of birds, which I dare say you have often seen flying high in the air; the difference, however, is that the flock of meteors is enormously greater than any flock of birds ever was; and the meteors, too, are scattered so widely apart, that each one may be miles away from its next neighbors. Usually the meteoric shoal is many millions of miles long, and perhaps a hundred thousand miles in width. The great flock of meteors travels through space in a certain definite track. We have learned how the sun guides a planet, and forces the planet to move around him in an ellipse. But our sun will also condescend to guide an object no bigger than a shooting star. A303 bullet, a pea, or even a grain of sand will be held to an elliptic course around the sun as carefully as the great Jupiter himself. The entire shoal of meteors may therefore pursue their common journey around the sun as if inspired by a common purpose, each individual member of the host being, however, guided by the sun, and performing its path in real independence of its neighbors. The orbit followed by this shoal of meteors is enormously large and wide. Here is a sketch of the path (Fig. 80), and I have laid down the position of the orbit of the earth, but not on the same scale. The ellipse is elongated, so that while the shoal approaches comparatively close to the sun at one end of its journey, at the other end it goes out to an enormous distance, far beyond the orbit of the earth—beyond, indeed, the orbit of Jupiter or Saturn; in fact, it reaches to the304 path of Uranus. To accomplish so vast a journey as this thirty-three years and a quarter are required, and now you will be easily able to see why we get periodical visits from the shoal.

It is, however, a mere piece of good fortune that we ever encounter the November meteors. Probably there are numerous other shoals of meteors quite as important which we never see, just in the same way as there are many shoals of fish in the sea that never come into our net. The earth moves round the sun in a path which is very nearly a circle, and the shoal moves round in this long oval. We cannot easily represent the true state of things by mere diagrams which show all these objects on the same plane. This does not give an accurate representation of the orbits. I think you will better understand what I mean by means of some wire rings. Make a round one to represent the path of the earth, and a long oval one to represent the path of the meteors. There is to be a small opening in the circular ring so that we can slip one of the orbits inside the other. If we are to see the meteors, it is of course necessary that they should strike the earth’s atmosphere, for they are not visible to us when they lie at a distance like the moon or like the planets. It is necessary that there be a collision between the earth and the shoal of meteors. But there never could be a collision between two trains unless the lines on which these trains run meet each other; therefore, it is necessary that this long ellipse shall actually cross the earth’s track; it will not do to have it pass inside like the two links of a chain; our earth would then miss305 the meteors altogether, and we should never see them. There are very likely many of such shoals of meteors revolving in this way, and thus escaping our notice entirely.

You will also understand why there is no use in looking for these showers except on a particular day of November. On that day, and on that day alone, the earth appears at that particular point of its route where the latter crosses the track of the shoal. On the 1st of November, for instance, the earth has not yet reached the point where it could meet with these bodies. By the end of November it has passed too far. But even supposing that the earth is crossing the track of the meteors on the 13th of November, it is still possible that only a few, or none at all, shall be seen. The shoal may not happen to be at that spot at the right time. For a display of meteors to occur, it is therefore necessary that the shoal shall happen to be passing this particular stage of the journey on the 13th of November. In 1866 the earth dipped through the shoal and caught a great many of these meteors in its net. For a few hours the earth was engaged in the capture, until it emerged on the other side of the shoal, and the display was at an end.

Sometimes it happens that in two years following each other, grand showers of meteors are seen. The reason of this is that the shoal is very long and thin, and consequently if the earth passes through the beginning of the shoal one year, it may have returned to the same point next year before the whole length of the shoal has completely passed. In this case we shall306 have two great showers in consecutive years. Thus a very fine display was seen in America on the proper day in 1867, while many stragglers were also observed during the three subsequent recurrences of the same date.

Whenever the 13th of November comes round we generally meet with at least five shooting stars belonging to this same system, and we must explain how this occurs. Suppose there is a small racecourse so that the competitors will have to run a great many times round before the race is over. Let there be a very large number of entries, and let the majority of the athletes be fairly good runners, while a few are exceptionally good with varying degrees of excellence, and a few are very bad, some being worse than others. The whole group starts together in a cluster at the signal, and perhaps for the first round or two they may keep tolerably well together. It will be noticed the cluster begins to elongate as one circuit after another is made; the better runners draw out to the front, and the slower runners lag further and further behind; at last it may happen that those at the head will have gained a whole round on those at the tail, while the other runners of varying degrees of speed will be scattered all round the course. The majority of the runners, if of nearly equal speed, may continue in a pretty dense group.

Precisely similar has been the great celestial race which these meteors are running. They started on their grand career centuries ago, and ever since then they have been flying round and round their mighty course. The greater proportion of the meteors still307 stay close together, and their pace is nearly uniform. The exceptionally smart ones have shot ahead, the exceptionally slow ones have lagged behind, and thus it happens that, after fifty or more revolutions have been completed, the shape of the original swarm has become considerably modified. Its length has been drawn out, while the stragglers and the fastest runners have been scattered all around the path. Across this course our earth carries us every November; there we usually encounter some of the members of this swarm which have strayed from the great host; they flash into the air, and thus it is that some of these bodies are generally seen every November.

Fig. 81.—The Radiant.

During a shooting-star shower it is interesting to notice that all the meteors seem to diverge from a single point. In the adjoining figure (Fig. 81), which shows the directions of a number of meteors’ tracks, you will notice that every one seems to radiate from a certain point of the sky. In the case of the shower of the 13th–15th of November this point lies in the constellation Leo. I must refer you to the Appendix for a description of the way to find Leo or the Lion. The radiant point, as we term it, of this system of meteors is there situated. It is true that the meteors themselves do not generally seem to come all the way from this place. It is the direction of their luminous trails produced backward that carries the eye to the radiant (Fig. 81). If a meteor were actually seen there, it would be certainly coming straight towards us; it would not then appear as a streak of light at all; it would merely seem like a star which suddenly blazed into splendor and308 then again sank down into invisibility. Every meteor which appeared near this point would be directed very nearly at the observer, and its path would therefore seem very much foreshortened. I can illustrate this with a long straight rod. If I point it directly at you, you can only see the end. If I point it nearly at you, it will seem very much shortened. During the great shower in 1866 many of the meteors could be observed so close to the radiant in Leo that they seemed merely like very short marks in the sky; some of them, indeed, seemed to be merely starlike points swelling up into brilliance and then going out. Hence it is that we call this system of shooting stars the “Leonids.” They bear309 this name because their radiant lies in the constellation Leo, and unless the direction of a shooting star emanates from this point it does not belong to the Leonids. Even if it did so, the meteor would not be a Leonid unless the date was right, namely, on the 13th of November, or within a day thereof. We thus have two characteristics which belong to a system of shooting stars; there is the date on which they occur and the point from which they radiate.

OTHER GREAT SHOWERS.

To illustrate what I have said, we will speak about another system of shooting stars; they are due every August, from the 9th to the 11th, and their directions diverge from a point in the constellation of Perseus. I may remind you of the dates of the recurrence of this shower as well as of the November meteors of which we have just spoken, by quoting the following production:—

If you November’s stars would see,

About the fourteenth watching be.

In August, too, stars shine through heaven,

On nights between nine and eleven.

It may be worth your while to remember these lines, and always to keep a look-out on the days named. The August meteors, the Perseids we often call them, do not make gorgeous displays, in particular years, with the regularity of the Leonids. There have been, no doubt, some exceptionally grand showers between the 9th and the 11th of August, but we cannot predict310 when the next splendid one is due. There are vast numbers of stragglers all round the track of the Perseids. In fact, it would seem as if the great race had gone on for such a long period that the cluster had to a great extent broken up, and that a large proportion of the meteors were now scattered the whole way around the course with tolerable uniformity. This being so, it follows that every time we cross the track we are nearly certain to fall in with a few of the stragglers, though we may never enjoy the tremendous spectacle of a plunge through a dense host of meteoroids.

There are many other showers besides the two I have mentioned. Some shooting stars are to be seen almost every fine night, and those astronomers who pay particular attention to this subject are able to make out scores of small showers which might not interest you. Each of these is fully defined by the night of the year on which it occurs and the position of the point in the heavens from which the meteors radiate. Of these I must mention one. It is not usually very attractive, but it has a particular interest, as I shall now explain.

On the 27th of November, 1872, a beautiful meteoric shower took place. You will notice that though the month is the same, the day is entirely different from that on which the Leonids appear. This shower of the 27th is called the Andromedes, because the lines of direction of the shooting stars of which it is composed seem to diverge from a point in the constellation Andromeda. Ordinarily speaking, there is no special display of meteors connected with the annual return of this day; but in 1872 astronomers were astonished by311 an exhibition of shooting stars belonging to this system. They were not at all bright when compared with the Leonid meteors. They were, however, sufficiently numerous to arrest the attention of very many, even among those who do not usually pay much attention to the heavens.

The chief interest of the shower of Andromedes centers in a remarkable discovery connecting meteors and comets. There is a comet which was discovered by the astronomer Biela. It is a small object, requiring a telescope to show it. This comet completes each revolution in a period of about seven years; or rather, I should say, that was the time which the comet used to spend on its journey, for a life of trouble and disaster seems of late to have nearly extinguished the unfortunate object. In 1872 the comet was due in our neighborhood, and on the night of the 27th of November, in the same year, the earth crossed the track, and, in doing so, the shower of shooting stars was seen. This was a remarkable coincidence. We crossed the path of the comet at the time when we knew the comet ought to be there; and though we did not then see the comet, we saw a shower of shooting stars, and a wonderful shower too. A circumstance so peculiar suggested at once that the comet and the shooting stars must in some way or other be connected together. This is a suggestion we can test in another manner. We know the history of the comet, and we are aware that at the very time of the shower, the comet was approaching from the direction of the constellation of Andromeda. It was coming, in fact, from the very quarter whence312 the shooting stars have themselves travelled. Taking all these things together, it seems impossible to doubt that the shoal of shooting stars was, if not actually the comet itself, something closely connected with that famous body.

METEORITES.

Some years ago, a farmer living near Rowton, in Shropshire, noticed on a path in a field a hole which had been suddenly made by some mysterious and unknown agent. The laborers who were near told him they had just heard a remarkable noise; and when the farmer put his hand down into the hole, he felt something hot at the bottom of it. He took a spade and dug up the strange body, and found it to be a piece of iron, weighing about seven pounds. He was naturally amazed at such an occurrence, and brought the body home with him.

Where did that piece of iron come from? It is plain that it could not have been always in the ground. The noise and the recently made hole showed that was not the case, and that is confirmed by the fact that the iron was hot. A piece of iron within a few feet of the earth’s surface cannot have remained warm for any length of time. It is therefore clear that the iron must have tumbled from the sky. This is a marvellous notion; in fact, it seems so incredible that at first people refused to believe that such things as stones or solid lumps of iron could have fallen from the heavens to the earth. But they had to believe it; the evidence was too313 conclusive. Fortunately, however, the occurrence is a comparatively rare one; indeed, our life on this globe would have an intolerable anxiety added to it if showers of iron hailstones like that at Rowton were at all of frequent occurrence. We should want umbrellas of a more substantial description than those which suffice for the rains we actually experience. There are, indeed, instances on record of persons having been killed by the fearful blows given by these bodies in falling.

The Rowton siderite is a comparatively small one; pieces weighing hundredweights, and even tons, have been collected together in our museums. I would recommend you to pay a visit to that interesting room in our great British Museum in which these meteorites are exhibited. There we see actual specimens of celestial bodies which we can feel or weigh, and which our chemists can analyze. It may be noticed that they only contain substances that we already know on this earth. This celestial iron has often been made use of in primitive times before man understood how to smelt iron from its ore and how to transform it from cast iron to wrought iron. Nature seems to have taken heed of their wants, and occasionally to have thrown down a lump or two for the benefit of those who were so fortunate as to secure them.

That these stones or irons drop from the sky is absolutely certain, but when we try to find out their earlier history we become involved in not a few difficulties. Nobody really knows the true history of these objects, but the view of their origin which seems to me to possess314 fewer difficulties than any other view is that which we may call the Columbiad Theory. I use this expression because every boy or girl listening to me ought to have read Jules Verne’s wonderful book, “From the Earth to the Moon,” and if any of you have not read it, the sooner you do so the better. It is there narrated how the gun club of Baltimore designed a magnificent cannon which was sunk deep into the ground, and then received a terrific charge of guncotton, on which a great hollow projectile was carefully lowered, containing inside the three adventurous explorers who desired to visit the moon. Calculations were produced with a view of showing that by firing on a particular day the explosion would drive the projectile up to the moon. There was, however, the necessary condition that the speed of projection should be great enough. The gun club were accurate in saying that if the cannon were able to discharge the projectile with a speed twenty or thirty times as great as that which had ever been obtained with any other cannon, then the missile would ascend up and up forever if no further influence were exerted on it. No doubt we have to overlook the resistance on the air and a few other little difficulties, but to this extent, at all events, the gun club were right: that a velocity of about six or seven miles a second would suffice to carry a body away from the gravitation of the earth.

No one supposes that there were ever Columbiad cannons on our globe by which projectiles were shot up into space; but it seems possible that there may have been in very ancient days volcanoes on the earth315 with a shooting power as great as that which President Barbicane designed for the big cannon.

Even now we have some active volcanoes of great energy on our earth, and we know that in former days the volcanoes must have been still more powerful; that, in fact, the Vesuvius of the present must be merely a popgun in comparison with volcanoes which have shaken the earth in those primitive days when it had just cooled down from its original fiery condition. Some of these early volcanoes, in the throes of their mighty eruptions, appear to have shot forth pieces of iron and volcanic substances with a violence great enough to carry them off into space.

Suppose that a missile were projected upwards, it would ascend higher and higher, and gravity would, of course, tend to drag it back again down to earth. It can be shown that with an initial speed of six or seven miles a second the missiles would never return to the earth if only influenced by its attraction. The subsequent history of such a projectile would be guided by the laws according to which a planet moves. The body is understood to escape the destination which was aimed at by the Columbiad. I mean, of course, that it is not supposed to hit the moon. Of course, this might conceivably happen; but most of the projectiles would go quite wide of the mark, and would travel off into space.

Though the earth would be unable to recall the projectile, the attraction of the sun would still guide it, whether it was as big as a paving-stone or ever so much larger or smaller. The body would be constrained to316 follow a path like a little planet around the sun. This track it would steadily pursue for ages. The wanderer would, however, cross the earth’s track once during each of its revolutions at the point from which it was projected. Of course, it will generally happen that the earth will not be there at the time the meteorite is crossing, and the meteorite will not be there at the time the earth is crossing. Nothing will therefore happen, and the object goes again on its long rounds. But sometimes it must occur that a meteor does not get past the junction without coming so close to the earth that it plunges into the air, often producing a noise and generating a streak of light like a shooting star. Then it tumbles down, and is restored to that earth whence it originally came.

If this be the true view—and I think there are less weighty objections to it than to any other I know of—then the history of the piece of iron that was found in Shropshire would be somewhat as follows. Many millions of years ago, when the fires of our earth were much more vigorous than they are in these dull times, a terrific volcanic outbreak took place, and vast quantities of material were shot into space, of which this is one of the fragments. During all the ages that have since elapsed this piece of iron has followed its lonely track. In a thousandth part of the time rust and decay would have destroyed it had it lain on the earth, but in the solitudes of space there was found no air or damp to produce corrosion. At last, after the completion of its long travels, it again crashed down on the earth.

We have now briefly surveyed the extent of the solar317 system. We began with the sun, which presides over all, and then we discussed the various planets with their satellites, next we considered the eccentric comets, and finally the minute bodies which, as shooting stars or meteorites, must be regarded as forming part of the Sun’s system. In our closing lecture we shall have to deal with objects of a far more magnificent character.

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